1. Technical Field
The present invention relates to transmit diversity and reception equalization in a mobile communication system for reducing required transmitted power needed to achieve reliable communication.
2. Description of the Background Art
In duplex radio systems such as cellular telephone systems including a forward link and a reverse link, link balance must be maintained to ensure overall communication quality. Typically, reverse link receiver systems at a cellular base station employ diversity reception with two or more reception antennas spread 7-10xcex so that fading of mobile station transmission as perceived by the base station can be mitigated. However, multiple antennas and receiver channels are not feasible for vehicle mounted or hand-held mobile communication devices in which small size and reduced cost are important. Since vehicle mounted or hand-held mobile communication devices can not employ reception diversity, uplink performance is typically 6-7 dB better than downlink performance. Conventionally, link balance is maintained by using a stronger base station downlink transmission power amplifier to make up for the lack of diversity reception at the mobile receiver to thus improve downlink performance. However, increased power transmission has negative impact on link power budget, component size, weight and cost and also results in increased system interference.
FIG. 1 illustrates a conventional mobile communication system including base station 60 having a single base station transmit antenna 601 that wirelessly transmits a signal to mobile station 70 having antenna 701. Due to environmental obstacles such as buildings, trees or mountains located between mobile station 70 and base station 60, a signal transmitted from base station 60 will be received at mobile station 70 along with a plurality of multipath signals which are delayed in time after reflecting off various obstacles. FIG. 2 illustrates multipath delay of the received signal due to environmental obstacles. An adaptive equalizer within mobile station 70 has variable magnitude weightings and time offsets to compensate for changes in channel response due to motion of the mobile station which changes the geometry of signal reflections in the environment. Upon receiving a signal, the equalizer delays the multipaths of the received signal in an attempt to flatten the received channel response to compensate for radio channel distortions created by multipath. The equalizer functions in the frequency domain to adaptively mitigate the smearing of the multipaths.
In North American time division multiple access (TDMA) systems, which transmit narrow band signals of 30 kHz, the bit period is very long and the equalizer taps of the mobile station equalizer are separated by xc2xc to 1 bit, which corresponds to multipath echoes from great distances. Since propagation delays from multipaths due to environmental obstacles are relatively short (typically xc2xc of an information bit), mobile station equalizers in TDMA systems do not effectively mitigate multipaths caused by environmental reflections because most of the multipath is within delays that are too short for the equalizer to handle. In general, since mobile station equalizers in TDMA systems can not effectively mitigate multipath smearing, the equalizers are thus usually maintained in a differential mode (equalizer OFF). On the other hand, equalizer receivers in mobile stations of GSM (Global System for Mobile Communications) systems and RAKE receivers in mobile stations of CDMA (code division multiple access) systems may significantly mitigate multipath. However, the configuration of the equalizer receivers and RAKE receivers for GSM and CDMA systems are complex.
FIG. 3 illustrates the effects of conventional diversity reception as plotted in terms of depth of fade with respect to fading probability. For example, in the case of one-branch reception using a single antenna, ten percent of the time the signal fade is 25 dB or more. However, in the case of two-branch reception in which two signals are received using two different, independent antennas that are spatially separated at the base station such that the signal as received at the two antennas does not fade simultaneously, ten percent of the time the signal fade is 15 dB of more. In the case of four-branch reception using four antennas, ten percent of the time the signal fade is 10 dB or more. A diversity gain of 10 dB is therefore realized for two-branch reception in contrast to one-branch reception using the same transmitted signal strength. The fade margin is less for two-branch reception in this case and link budget can therefore be conserved since a given reception criteria can be realized using lower signal strength and multiple reception branches. However, reception diversity at a mobile station is impractical since typical hand-held or portable mobile units cannot include multiple antennas that are spatially separated.
The present invention improves downlink performance in a mobile communication system without increasing base station transmit power by employing base station transmitter diversity combined with mobile station reception equalization. A multichannel transmitter including multiple transmit antennas transmits a signal and one or more additional independent versions of the same signal with time delay to a mobile station. The energy of the independent received versions of the signal are equalized in the frequency domain at the mobile station using an equalizer or synchronized in time in a RAKE receiver to produce a composite signal. The diversity gain effect may thus be achieved so that the fading immunity margin of the system is increased, less total transmitted power is required and generated interference is reduced.